A high-resolution magazine scan...
The hump I suspect might be a natural circulation reactor they might be testing.
Natural circulation reactors, which uses less pumps and the difference of hot-cold in the water temperature to move the water around, would have been tall for gravity to aid downward water flow.
Example of one.
My hypothesis is that the reactor they are testing is so tall that it exceeds the draft of the submarine and a hump is needed to clear the top of it. Given the presence of a hump, and the potential water flow turbulence it would bring, the reactor and the hump would have to behind the sail and then streamlined to minimize the flow effects.
Not a problem for the 094 which can hide the height with its hump.
Next generation PLAN SSN would have to call for a taller draft on the sub in order to sufficiently contain the natural circulation reactor.